Cathodic process for the preparation of tetraalkyl lead compounds

ABSTRACT

THE PRESENT INVENTION RELATES TO THE PROCESS FOR THE PREPARTION OF TETRAALKYL LEAD COMPOUNDS WHEREIN THE ANOLYTE COMPRISES A SOLUTION OF TETRABUTYL AMMONIUM BROMIDE OR TETRABUTYL PHOSPHONIUM BROMIDE IN AN ORGANIC SOLVENT. THE ANOLYTE MAY ALSO CONTAIN SOME WATER. IN THIS PROCESS BROMINE IS PRODUCED AT THE ANODE AND IS SOLUBLE IN THE ORGANIC SOLVENT PRESENT IN THE ANOLYTE. THIS BROMINE CAN BE RECOVERED BY TREATING THE ANOLYTE SOLUTION WITH HYDROGEN SULFIDE TO CONVERT THE BROMINE PRESENT TO HYDROGEN BROMIDE AND SULFUR. THE SULFUR IS REMOVED FROM THE SOLUTION AND THE SOLUTION IS TREATED WITH ETHANOL TO FORM ETHYLBROMIDE BY REACTION WITH THE HYDROGEN BROMIDE PRESENT IN THE SOLUTION. THE ETHYLBROMIDE IS SEPARATED FROM THE SOLUTION AND RETURNED TO THE CATHODE COMPARTMENT OF THE ELECTROLYTIC CELL WHEREBY IT IS SUBSEQUENTLY CONVERTED TO TETRAETHYL LEAD. THE QUATERNARY BROMIDE SALT AND THE ORGANIC SOLVENT IS RETURNED TO THE ANODE COMPARTMENT OF THE ELECTROLYTIC CELL WHERE IT THEN BECOMES PART OF THE ANOLYTE.

United States Patent US. Cl. 204-72 3 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to the process for the preparation of tetraalkyl lead compounds wherein the anolyte comprises a solution of tetrabutyl ammonium bromide or tetrabutyl phosphonium bromide in an organic solvent. The anolyte may also contain some water. In this process bromine is produced at the anode and is soluble in the organic solvent present in the anolyte. This bromine can be recovered by treating the anolyte solution with hydrogen sulfide to convert the bromine present to hydrogen bromide and sulfur. The sulfur is removed from the solution and the solution is treated with ethanol to form ethylbromide by reaction with the hydrogen bromide present in the solution. The ethylbromide is separated from the solution and returned to the cathode compartment of the electrolytic cell whereby it is subsequently converted to tetraethyl lead. The quaternary bromide salt and the organic solvent is returned to the anode compartment of the electrolytic cell where it then becomes part of the anolyte.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates generally to a cathodic process for the preparation of tetraalkyl lead compounds but more particularly to a process where the bromine generated at the anode is subsequently recovered as ethylbromide and returned to the cathode compartment of the electrolytic cell for production of additional tetraalkyl lead compounds.

Description of the prior art Cathodic processes for the production of alkyl leads are known. For example, Calingaert (US. Pat. No. 1,539,297) and Mead (US. Pat. No. 1,567,159) disclose tetraalkyl lead formation at a lead cathode by eleetrolyzing a catholyte consisting of an alkyl iodide in either alcoholic caustic or aqueous caustic containing casein. Also Silversmith et al. (US. Pat. No. 3,197,392) discloses a process for preparing tetraalkyl lead compounds by electrolyzing at a lead cathode a solution of an alkyl halide in a normally liquid nonhydroxylic cathodic solvent. In addition, a recent disclosure in this field is a patent to Smeltz (US. Pat. No. 3,392,093) which discloses a process for producing tetraalkyl lead compounds at a lead cathode in an electrolytic cell having a catholyte containing acetonitrile as a solvent, alkyl halide as an alkylating agent, a tetraalkyl ammonium monohalide as a current carrier and from about 1 to about 20 moles of water per mole of said current carrier added as a side product suppressor. It is also known to utilize a quaternary salt such as tetrabutyl ammonium bromide or tetrabutyl phosphonium bromide as a salt for use in the anolyte of a cathodic tetraalkyl lead process. These tetrabutyl ammonium and phosphonium bromides have been used to overcome the problem created by using a salt such as ammonium bromide whereby the ammonium ion was transferred from the anolyte compartment into the catholyte compartment and interferes 'With the cathodic reaction. This problem is avoided by using the tetrabutyl ammonium and phosphonium bromides because the size of the molecule retards its transference through the membrane into the cathode compartment. Even if a small amount of the tetrabutyl ammonium or phosphonium bromide salt happens to enter the cathode compartment from the anode compartment, it would not interfere with the cathodic reaction because this is the same salt which is used as a current carrier in the catholyte. There was a problem, however, with the use of these tetrabutyl ammonium and phosphonium bromides in that the bromide formed at the anode would form a precipitate with the tetrabutyl ammonium and phosphonium bromides when said quaternary salts were present in an aqueous solution and would result in a loss of bromine. The present invention provides a method of preventing the precipitation of bromine and for recovering said bromine from the anolyte and returning it as ethylbromide to the cathode compartment.

SUMMARY OF THE INVENTION The present invention relates to a method for recovering the bromine which is generated in the anode compartment of the electrolytic cell. This method involves the use of an organic solvent in the anolyte in which the bromine is soluble. This organic solvent can be present as a nonaqueous solution containing the quaternary ammonium or phosphonium bromide salts or it can also con tain water. The bromine can be recovered from the organic solvent by reacting with H 8 and ethanol to form ethylbromide which can be distilled from the solution and returned to the catholyte for subsequent conversion to tetraethyl lead. The remaining organic solvent and quaternary bromide salt can be returned to the anolyte compartment for subsequent recovery of additional bromine. It is therefore a general object of the invention to provide the method for recovering bromine from the anode and returning this bromine as ethylbromide to the cathode campartment wherein the ethylbromide is subsequently converted to tetraethyl lead.

A further object of the invention is to provide a method for recovering the organic solvent and tetrabutyl ammonium bromide or tetrabutyl phosphonium bromide for subsequent use as a component of the anolyte of said electrolytic cell. Other and further objects will become apparent from the following detailed description of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Broadly described, the method of the present invention comprises an anolyte containing an organic solvent, a quaternary salt such as tetrabutyl ammonium bromide or tetrabutyl phosphonium bromide and can contain water. In the cathodic production of tetraalkyl lead compounds wherein the catholyte contains an alkylbromide, bromine is liberated at the anode. This bromine is dissolved in the organic solvent as the adduct of the quaternary salt (QNBr-Br an is subsequently treated with hydrogen sulfide and ethanol to convert the bromine present to ethylbromide. The ethylbromide is returned to the catholyte for subsequent production of additional tetraethyl lead. The organic solvent containing the quaternary ammonium bromide can he returned to the anolyte compartment for subsequent recovery of additional bromine. The anolyte can be a nonaqueous system where in the components are an organic solvent having dissolved therein a quaternary ammonium or phosphonium bromide salt. An alternate anolyte composition can be an organic solvent and water system containing quaternary ammonium or phosphonium bromide salts dissolved therein. The organic solvent present in said system dissolves any bromine adduct formed with the quaternary ammonium or phosphonium bromide salts which occurs in an aqueous system without the presence of the organic solvent. The amount of water present in said system is not critical. When the anolyte is a totally aqueous system the bromine forms a precipitate with the quaternary ammonium or phosphonium bromide salts present. When the anolyte system in an nonaqueous system containing an organic solvent and the quaternary ammonium or phosphonium bromide salts the electrical resistance of the system is of course much higher than is present when the system also contains water. In carrying out the process of this invention a balancing of these two factors can be used to create the desired effect. For example, where power costs are limited, it is desirable to use a system containing water. Where :power costs are not important, however, it is advantageous to use a nonaqueous system to avoid subsequent handling and separation problems which are inherent in any system containing an aqueous layer and an organic layer. Organic solvents suitable for use in this process include methylbromide, ethylbromide, chloroform, carbon tetrachloride, and other organic solvents in which the bromine adducts are soluble and which are not destroyed by the current flowing in said electrolytic cell. Ethylbromide is the preferred solvent for the reason that the bromine adducts are highly soluble in ethylbromide and in addition the ethylbromide is also a component of the catholyte. Upon subsequent treatment of the anolyte solution for recovery of the bromine the conversion of the bromine to ethylbromide simply creates additional ethylbromide for subsequent use in the process. For example, upon the addition of hydrogen sulfide and ethanol to the system containing bromine additional ethylbromide is formed and part of this ethylbromide can be removed by distillation and returned to the catholyte compartment and the remaining ethylbromide and quaternary ammonium and phosphonium bromide salts can be returned to the anolyte for subsequent use in recovering additional bromine. This avoids the problem which is present when other solvents are used of completely removing the ethylbromide from the organic solvent present prior to returning the solvent to the anolyte. In carrying out the processes of this invention it is also advantageous to add a surfactant to the system containing the organic solvent and water so as to form an emulsion thereby reducing the electrical resistance of the anolyte system. As previously stated, this process solves problems which are created by the use of tetrabutyl ammonium and phosphonium bromides as salts for use in the anolyte. It Was found that when aqueous solutions of ammonium bromide or alkali bro mide were used as the anolyte a significant fraction of the current was carried by the ammonium ions and the alkali metal ions into the cathode compartment by passing through the semi-permeable ion exchange organic membrane separating the anode and cathode compartments. This means a substantial waste of these cations and the loss of electricity because these ions are either reacted at the cathode or interfere with the cathodic reduction of tetraethyl lead. In an ideal process all the current would be carried by bromide ion migrating through the semipermeable membrane from the cathode to the anode compartment. It was also found that a quaternary salt such as tetrabutyl ammonium bromide or tetrabutyl phosphonium bromide could be used to alleviate the problems of the migrating ions present when ammonium bromide or alkali bromides are used. It was also found, however, that the quaternary salts formed a precipitate with the bromine which was liberated at the anode and resulted in the loss of bromine.

This invention then provides a means whereby the bromine is maintained in solution and is subsequently converted to ethylbromide which becomes a part of the catholyte and is subsequently converted to the desired 4 tetraethyl lead compound. The invention can be better understood by referring to the following examples.

Example 1 I Percent Lead loss eflicientcy 97 TEL efiiciency 83- Lead converted 86 A precipitate formed in the anolyte which was analyzed and found to be a tetrabutyl ammonium bromidebromine adduct (QNBr-Br The electrical resistance of the anolyte was 20 ohms/cm Example 2 The anolyte composition was 10 grams tetrabutyl ammonium bromide, 90 grams water, and 25 grams ethylbromide. The other conditions were the same as Example l. The following results were obtained.

Percent Lead loss efiiciency 97 TEL efliciency 83 Lead converted 86 The anolyte solution remaining after electrolysis contained an aqueous layer and an organic layer and there was no evidence of any precipitate in either layer. The electrical resistance of the anolyte was 20 ohms/circ Example 3 The anolyte composition was 10 grams tetrabutyl ammonium bromide and 90 grams ethylbromide. The other conditions were the same as Example 1. The following results were obtained.

Percent Lead loss efiiciency 97 TEL efficiency 83 Lead converted 86 The anolyte solution remaining after electrolysis was a single phase organic solution and there was no evidence of any precipitate in the solution. The electrical resistance of the anolyte was ohms/cmfi.

Example 4 The anolyte from Example 3, after electrolysis, was treated with H 5 to effect a 1:1 molar ratio reaction with the bromine in the anolyte. The HBr formed was reacted with ethanol in a 1:1 molar ratio. The resulting product was ethylbromide which was separated from the system by distillation. The ethylbromide was then returned to the cathode compartment of the electrolytic cell. The distillation can be carried out so as to remove only a portion of the ethylbromide and the solution remaining contains ethylbromide, tetrabutyl ammonium bromide and a small amount of water. The solution can then be returned to the anode compartment of the electrolytic cell.

Although certain specific embodiments of the invention have been described as exemplary of its practice these examples are not intended to limit the invention in any way. Other process parameters and materials may be used in accordance with broad principles outlined herein and when so used are deemed to be circumscribed by the spirit and scope of the invention except as necessarily limited by the appended claims and reasonable equivalents thereof.

What is claimed is:

1. A cathodic process for the preparation of tetra-alkyl lead compounds at a lead cathode in an electrolytic cell having a lead cathode, an anode, and a current-permeable partition separating the catholyte from the anolyte and where the catholyte comprises an alkylbromide and a current carrier of tetrabutyl ammonium bromide or tetrabutyl phosphonium bromide and an anolyte which comprises a current carrier of tetrabutyl ammonium bromide or tetrabutyl phosphonium bromide and an organic solvent of ethylbromide, methylbromide, chloroform, or carbon tetrachloride, including the additional steps of:

(a) treating the anolyte solution with hydrogen sulfide to convert any bromine present to hydrogen bromide; (b) contacting said solution from step (a) with ethanol to form ethylbromide; and (c) separating ethylbromide from the solution of step (b) and returning said ethylbromide to the cathode compartment of said electrolytic cell.

2. The process of claim 1 wherein the organic solvent in the anolyte is ethylbromide.

3. The process of claim 1 wherein the anolyte comprises water, an organic solvent of ethylbromide, chloroform, methylbromide, or carbon tetrachloride and a current carrier of tetrabutyl ammonium bromide or tetrabutyl phosphonium bromide.

References Cited UNITED STATES PATENTS 3,392,093 7/1968 Smeltz 204-72 JOHN H. :MACK, Primary Examiner N. A. KAPLAN, Assistant Examiner US. Cl. X.R. 20459 

